1. Technical Field
The present invention relates to a process for recovering liquid hydrocarbons from a production wellbore in fluid communication with a subterranean hydrocarbon-bearing formation having a gas cap overlying a liquid hydrocarbon producing zone, and more particularly to such a process wherein a polymer enhanced foam is placed in the formation to reduce gas coning into the production wellbore.
2. Description of Related Art
A specific problem frequently encountered during the recovery of liquid hydrocarbons from an oil-bearing producing zone of a subterranean formation having an overlying gas cap is a phenomenon termed "gas coning". This phenomenon occurs when there is fluid communication between the producing zone and the gas cap. Under sufficient drawdown pressure, the gas cap gas is drawn down and coproduced with the liquid hydrocarbons from the producing zone. Due to the higher mobility of the gas cap gas, the gas also tends to inhibit the flow of liquid hydrocarbons into the wellbore by preferentially entering the wellbore to the exclusion of the liquid hydrocarbons. Accordingly, gas coning is characterized by a significant increase in the gas oil ratio of the produced fluids and an attendant significant decrease in the liquid hydrocarbon recovery rate from the production wellbore.
Gas coning treatments generally rely on placement of a gas blocking agent in the formation between the gas cap gas and the liquid hydrocarbon production wellbore to substantially block vertical flowpaths for the gas into the production wellbore. U.S. Pat. No. 5,105,884 to Sydansk discloses the use of a conventional oilfield foam augmented with a crosslinked polymer gel as a gas blocking agent in vertical fractures extending between a gas cap and a liquid hydrocarbon producing zone affected by gas coning. Although the gel-containing foam is extremely effective for its intended purpose, application of the gel-containing foam has a distinct, albeit minimal, degree of operational complexity and risk. As with any chemical reaction, the crosslinking reaction between the crosslinking agent and polymer requires greater operator attention to process controls to ensure that the reaction proceeds properly. Furthermore, the strength of the resultant foam is such that the foam can be difficult to remove if it is improperly placed in the formation or if one subsequently desires to return the formation to its original permeability condition.
By comparison, conventional foams, typically consisting of only a surfactant solution and a foaming gas, are relatively cheaper and simpler to formulate than gel-containing foams and obviate certain operational complexities, being relatively easy to break down for removal from the formation and not requiring the added process controls and chemical cost of the crosslinking reaction. Nevertheless, the gel-containing foams are deemed superior to conventional oilfield foams for treating gas coning because the performance of conventional foams in the treatment of gas coning has been found inadequate.
Whereas, conventional oilfield foams generally perform satisfactorily as dynamic mobility control fluids in hydrocarbon displacement floods, different performance demands are required of a foam acting as a static gas blocking agent. In this role, conventional oilfield foams often do not have sufficient structural strength, stability, or critical pressure gradient for flow to achieve long-term performance demands when continuously subjected to the gas flow pressure gradient existing during gas coning. Furthermore, conventional oilfied foams often do not have sufficient stability to maintain their structural integrity when subjected to downhole formation conditions typically encountered, particularly in the presence of liquid hydrocarbons.
U.S. Pat. No. 5,129,457 to Sydansk discloses the use of foams augmented with an uncrosslinked polymer for specific hydrocarbon displacement flooding applications. In particular, such foams are disclosed as having utility as mobility-controlling hydrocarbon displacement fluids or as flow diverters for conventional hydrocarbon displacement fluids. Persoff, P. et al., "Aqueous Foams for Control of Gas Migration and Water Coning in Aquifer Gas Storage", Energy Sources, v.12, pp. 479-497, 1990, also suggests the use of a foam augmented with a polymer for the narrow application of stabilizing a foam used to block water coning in an underground gas storage aquifer. However, neither of these references recognizes or suggests the potential utility of a foam augmented with an uncrosslinked polymer to treat gas coning in an oil-bearing formation and to overcome the unique problems and complexities attendant therewith. In particular, it was not heretofore believed that a foam augmented solely with an uncrosslinked polymer had sufficient strength and stability to be effective in a gas coning treatment, particularly in the presence of a liquid hydrocarbon.
As is apparent from the above, a gas coning treatment process is needed which employs a gas blocking agent that couples the predictable effectiveness and reliability of a gel-containing foam with the operational simplicity and low cost of a conventional oilfield foam. Accordingly, it is an object of the present invention to provide an alternative gas blocking agent having utility in a gas coning treatment which overcomes the problems encountered with the use of known gas coning treatment agents. It is further an object of the present invention to provide an alternative blocking agent that is economical and operationally simple to apply, yet has a high degree of strength and stability when placed in a formation.